This invention relates to surface passivation methods for compound semiconductor microelectronic and optoelectronic materials. More particularly, it relates to the use of sulfur as the passivating substance which combines with the surface with the assistance of UV photolysis.
This photosulfidation process differs from existing technology in a very fundamental way. It is the first passivation process using elemental sulfur vapor combined with chemical activation of sulfur by photoexcitation using UV light to generate highly reactive S-species. This use of photon absorption and molecular excitation to generate reactive sulfur species has not previously been employed for passivation of compound semiconductor surfaces. There have been a number of previous passivation processes based on sulfur compounds, but none have employed photoactivation. These earlier processes include wet dips/spins of solutions of various sulfides such as Na.sub.2 S, (NH.sub.4).sub.2 S, (NH.sub.4).sub.2 S.sub.x, and P.sub.2 S.sub.5 /(NH.sub.4).sub.2 S, treatment with organic thiols, treatment with hydrogen sulfide plasmas, and spin-casting of solutions of As.sub.x S.sub.y. After consideration of the effects of semiconductor doping level and photoluminescence (PL) excitation wavelength, all these earlier treatments and this photosulfidation process produce the same order of magnitude change in PL intensity, indicating comparable efficacy as passivation processes. When properly handled, the present process can produce PL results.about.50%-75% better than the earlier treatments. This relative equivalence is not unexpected, since in all cases semiconductor-sulfur bonding is responsible for the improved electronic properties. Consequently, the following comparison of processes will emphasize the numerous advantages of our photosulfidation process in areas other than passivation.
There are some general advantages of this photosulfidation process over alternative S-based passivation processes. These include such features as negligible generation of hazardous waste, low toxicity of the chemicals involved, the convenience of transient exposure to UV light and a low pressure vapor, the inexpensive equipment requirements (a high vacuum reaction chamber and a UV light source such as a high-pressure Hg lamp), the low probability of process-induced damage to semiconductor, and flexibility regarding the point in the device fabrication process at which passivation can be performed, i.e., it can be done after metallization as a final process step before encapsulation. In addition, it can be readily incorporated as a step in vacuum integrated processing.